* Car'd on the foam. l. 61. The phenomena of the tides have been well investigated and satisfactorily explained by Sir Isaac Newton and Dr. Halley from the reciprocal gravitations of the earth, moon, and sun. As the earth and moon move round a centre of motion near the earth's surface, at the same time that they are proceeding in their annual orbit round the sun, it follows that the water on the side of the earth nearest this centre of motion between the earth and moon will be more attracted by the moon, and the waters on the opposite side of the earth will be less attracted by the moon, than the central parts of the earth. Add to this that the centrifugal force of the water on the side of the earth furthest from the centre of the motion, round which the earth and moon move, (which, as was said before, is near the surface of the earth) is greater than that on the opposite side of the earth, From both these causes it is easy to comprehend that the water will rise on two sides of the earth, viz. on that nearest to the moon, and its opposite side, and that it will be flattened in consequence at the quadratures, and thus produce two tides in every lunar day, which consists of about twenty-four hours and forty-eight minutes.
These tides will be also affected by the solar attraction when it coincides with the lunar one, or opposes it, as at new and full moon, and will also be much influenced by the opposing shores in every part of the earth.
Now as the moon in moving round the centre of gravity between itself and the earth describes a much larger orbit than the earth describes round the same centre, it follows that the centrifugal motion on the side of the moon opposite to the earth must be much greater than the centrifugal motion of the side of the earth opposite to the moon round the same centre, And secondly, as the attraction of the earth exerted on the moon's surface next to the earth is much greater than the attraction of the moon exerted on the earth's surface, the tides on the lunar sea, (if such there be,) should be much greater than those of our ocean. Add to this that as the same face of the moon always is turned to the earth, the lunar tides must be permanent, and if the solid parts of the moon be spherical, must always cover the phasis next to us. But as there are evidently hills and vales and volcanos on this side of the moon, the consequence is that the moon has no ocean, or that it is frozen.
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